Show hint to narrow to compose recipients when it differs (before sending)

[neiljp]

We've already implemented this as part of #1194 (first take of #1182) when one sends a message.

However, this could be dynamically displayed before sending, as per the button discussed in #design > button to go to conversation, and could be a button here too as long as focus is carefully preserved.

This hint may be as simple as the shortcut key (currently eg [meta .]), also with a phrase (go to conversation) or some other variant - bearing in mind this will take up potentially a large amount of compose box space, this needs some careful consideration.